1. Field of the Invention
The present invention relates generally to a liquid ejection printing apparatus which performs printing by ejecting liquid to form ejected liquid drops to deposit on a medium to be printed. More specifically, the invention relates to a liquid ejection printing apparatus having a preparatorily ejecting function and a printing method thereof. Here, the term printing includes providing ink on all types of ink support, such as cloths, yarns, paper, sheet members and so forth. The definition of printing apparatus covers a variety of information processing apparatuses or a printers as an output device therefor. The present invention is suitable for those applications.
2. Description of Related Art
In a liquid ejection printing apparatus performing printing by ejecting liquid, it has been known to cause ejection failure, in which a liquid droplet cannot be ejected despite the fact that a normal ejection signal is transmitted upon initiation of printing, when the printing apparatus has been left without performing printing for a long period.
As a major cause of occurrence of initial ejection failure, the following two causes are considered.
The first cause is lowering of the temperature of liquid under low temperature environment. Associated with this, viscosity of the liquid may be increased. Due to viscous resistance of the liquid, required energy for liquid ejection can be increased to cause ejection failure due to lack of ejection energy.
A second cause is increasing of viscous resistance due to increasing of viscosity of the liquid resulting from evaporation of the liquid at ejection openings. This is significant in a low temperature and low humidity environment and will cause a serious problem in the liquid ejection printing system.
As one of the measures to resolve this, there is a method using a preliminary ejection system. This system automatically performs ejection of liquid toward a liquid absorbing body and so forth a reasonable number of times upon onset of a power supply or when printing has not been performed for a long period, for ejecting out liquid having high viscosity and thus for lowering the viscosity of the liquid. During this preliminary ejection, liquid ejection failure can be resolved to obtain normal ejection upon printing.
Also, pre-heating in advance of ejection has also been performed. For instance, Japanese Patent Application Laid-Open No. 2765/1982 discloses a printing apparatus which can lower viscosity of ink by heating high viscous ink within an ejection head immediately before initiation of printing.
Also, Japanese Patent Application Laid-Open No. 146548/1986 (U.S. Pat. No. 5,339,098) discloses a method, in which a dedicated controller for controlling ejection from a head unit is provided and a pre-heating process and a preliminary ejection process are performed employing such dedicated controller.
Furthermore, Japanese Patent Application Laid-Open No. 116153/1987 discloses means for making a printing heat to perform non-printing ejection of a liquid printing material at a position outside of a printing region depending upon environmental conditions of a place where the apparatus is located.
Conventionally, an ejection drive frequency for preliminary ejection is set a fixed value which is lower than or equal to a frequency in actual printing and thus cannot be varied. This creates problems set out below.
For instance, in the case of an apparatus employing a thermal ink jet head, it may be possible that no liquid droplet is ejected with the first drive signal upon performing preliminary ejection. One of the causes may be a failure of generation of a bubble on a heater. In such case, for recovery to a printing operation, it becomes necessary to forcedly remove printing liquid having increased viscosity by way of a suctioning operation or so forth to introduce printing liquid in which bubbles can be generated from a supply system. It may be also possible that a liquid droplet cannot be formed, though a bubble is generated on the heater. In addition, even when a liquid droplet can be ejected at the first drive signal, it may be possible that the viscosity of printing liquid cannot be lowered unconditionally. Namely, when the second drive signal is applied before disappearance of the bubble generated by the first drive signal, behavior of the bubble becomes unstable to possibly reside as a fixed bubble, making subsequent preliminary ejection completely impossible. In general, high viscous ink takes a longer period for a bubble to disappear compared with low viscous ink. Therefore, the foregoing problem is likely to be caused at first and second ejection.
As shown in FIG. 10, with experimental head 03 in which heater 02 of 18 .mu.m.times.24 .mu.m is disposed in a straight tube form liquid passage 01 having a liquid passage length of 200 .mu.m and a liquid passage cross section of 20 .mu.m.times.20 .mu.m , water/glycerine solution is supplied to a liquid chamber 04 through a supply tube 05. Then, a life of a bubble generated on heater 02 was measured. The result of measurement is shown in FIG. 11. As shown in FIG. 11, it was appreciated that the life of the bubble is abruptly expanded according to increasing of viscosity of the liquid.
Accordingly, when a printing liquid droplet is ejected by the first drive signal of preliminary ejection, the following problem may arise. As set forth above, the liquid having increased viscosity has increased viscous resistance. As set forth above, since the liquid having increased viscosity has increased viscous resistance, after first ejection at a drive frequency of normal printing in preliminary ejection, if the subsequent second, third and fourth drive signals are applied before the meniscus of the ejection opening is completely resumed, the meniscus becomes quite unstable so as not to successfully eject the liquid with the increased viscosity. In particular, under low temperature and low humidity environment, the viscosity of the liquid is further increased to further increase viscous resistance to make it difficult to successfully perform preliminary ejection.
It can be a solution for the problem set forth above to perform preliminary ejection with a constant frequency lower than the drive frequency in normal driving. With this method, after meniscus recovers after the first ejection, it may be possible to gradually lower viscous level in the liquid ejection printing apparatus toward normal viscosity by carrying out sequentially the second, third and fourth ejections of high viscous liquid. However, since the preliminary ejection is continued at a constant low frequency even when the viscosity of the liquid is lowered to be close to the normal viscosity of the normal liquid, it takes a long period of time in the preliminary ejection. This results in delay in entry into actual printing operation.
In particular, in a thermal ink-jet head for high density printing, since the liquid passage is fine, particularly after leaving non-printing, the life of the bubble becomes extraordinarily long, e.g. several tens msec to several hundreds msec. The drive frequency to successfully and normally perform the preliminary ejection with such head becomes approximately 1 Hz. In such head, if the preliminary ejection is performed at the constant frequency of approximately 1 Hz, it may inherently take several minutes.
Moreover, the drive frequency in the preliminary ejection tends to be significantly influenced by the environmental condition in use. The drive frequency of the print head in the preliminary ejection is normally set low in view of severe condition. Accordingly, such setting is effective under bad environmental condition, but under good environmental condition where ejection failure is unlikely to be caused, the preliminary ejection has to be performed at the excessively low constant frequency, resulting in prolonging the preliminary ejection period.